(1) Field of the Invention
This invention relates to a method for fabricating a Group III nitride film, an underlayer for fabricating a Group III nitride film and a method for fabricating the same underlayer, particularly usable for semiconductor films constituting a light-emitting diode or a high velocity IC chip.
(2) Related Art Statement
Group III nitride films are employed as semiconductor films constituting light-emitting diodes, and recently, have received much attention as semiconductor films constituting high velocity IC chips for use in cellular phones.
Such Group III nitride films are usually fabricated by MOCVD methods. Concretely, a substrate on which Group III nitride films are formed is set onto a susceptor installed in a given reactor and then heated to 1000° C. or over with a heater provided in or out of the susceptor. Thereafter, raw material gases are introduced with a carrier gas into the reactor and supplied onto the substrate.
On the substrate, the raw material gases are dissolved through thermochemical reaction into constituent elements, which are reacted to deposit and fabricate a desired Group III nitride film on the substrate.
There are few suitable substrates which have lattice constants and thermal expansion coefficients nearly equal to those of Group III nitride films. In this case, the difference in the lattice constants between the Group III nitride film and the substrate may induce more misfit dislocations at the boundary between the Group III nitride film and the substrate. In order to reduce such misfit dislocations, generally, a lower crystallinity buffer layer, which is fabricated at a lower temperature, is provided between the substrate and the Group III nitride film. In this case, the difference in lattice constant is compensated, and thus, many misfit dislocations are not created.
However, the misfit dislocations are not reduced sufficiently even though the buffer layer is provided, so that many dislocations of about 109–1010/cm2 may be created in the Group III nitride film, originating from the propagation of the misfit dislocations. As a result, the crystal quality of the Group III nitride film is degraded, and thus, the electrical and optical properties of the Group III nitride film are degraded.
In order to solve the above problem, attempts have been made to form a patterned mask made of SiO2 on the substrate and epitaxially grow the Group III nitride film laterally on the substrate. In this case, misfit dislocations are propagated laterally on the mask, and not propagated vertically. Therefore, the dislocation density of the Group III nitride film can be lowered in the region above the patterned mask.
However, since photolithography processes including etching steps are required in forming patterned masks, the total number of steps in the fabricating process for the Group III nitride film is increased and complicated.